1. Field of the Invention
The present invention relates to a fixing device and an image forming apparatus including the fixing device.
2. Description of the Related Art
Image forming apparatuses typically employ a fixing device configured to perform a fixing process, in which a toner image carried on a recording sheet such as a sheet of paper is fused and fixed onto the sheet.
For example, JP-2005-156769-A discloses a belt-type fixing device including an endless rotary fixing belt looped around a plurality of rollers, opposed by a pressure roller to form a nip for fixation by contacting an outside of the fixing belt. Inside the fixing belt, there are at least a fixing roller and a heating roller. The fixing roller contacts the internal surface of the fixing belt and rotates while sandwiching the fixing belt together with the pressure roller that contacts the external surface of the fixing belt. The heating roller rotates while heating the fixing belt.
Normally, the fixing device includes a temperature detector such as a thermistor to detect a temperature of the surface of the fixing belt. The fixing device is controlled so that the fixing belt is rotated as the temperature detector detects the surface temperature of the fixing belt. The recording sheet is conveyed to the fixing nip upon the detected fixing belt surface temperature reaching a predetermined temperature.
In the type of the fixing device used in high-speed image forming apparatuses for bulk printing, a fixing nip having a length of 15 to 25 mm needs to be formed to secure a longer fixing time. For that purpose, a fixing roller with a larger diameter is used to form a greater nip, and a bite amount of the fixing roller into the opposed pressure roller needs to be greater. However, a distortion of the rubber layer of the fixing roller needs to be minimized even when the bite amount of the fixing roller into the pressure roller is increases. Accordingly, the fixing device for high-speed image forming apparatuses requires a fixing roller including a rubber layer with a greater depth.
As a result of greater depth of the rubber layer of the fixing roller, however, thermal capacity of the fixing roller increases and the fixing roller may be cooler than the surface of the fixing belt even through the fixing belt itself is sufficiently heated. When the temperature of the fixing roller does not reach a predetermined target temperature, the heat of the fixing belt is lost via not only the recording sheet but also the fixing roller at the fixing nip. Further, because the thermal expansion amount of the fixing roller is low, the nip pressure is decreased and the nip length shortened. As a result, the toner image on the recording sheet is not subjected to the necessary heat and pressure, resulting in defective imaging.